Tweak the Progress class to be simpler. Given that progress does
not need to support arbitrary depth but only has a single level
the class now just exposes "sub_progress" to the caller.
When the main progress is advanced the sub_progress is now fully
deleted instead of just reset. The rational is that when the main
progress is done and advances a step it is very likely that a
new sub_progress is required and it's most likely an error if
the same sub_progress will get re-used.
This means that `reset()` can be removed as it's not used anymore
(and YAGNI). We can add it back when we have a use-case.
It also change the code so that "total" starts with 0 instead
of `None` (principle of least surprise). This means that now
`progress.incr()` is called in the JSONSeqMonitor() for
`finish()` and `result()` to indicate that the pipeline/stage
is finished.
3fbd0b2a73